The present invention relates to an atomic clock that can be used as a high precision frequency standard. Ramsey's method of separated oscillatory fields has been utilized to make an atomic clock. This method aligns the quantum mechanical spin of a number of atoms, allows the atoms to precess, measures the precession and then uses this measurement to calibrate an external oscillator.
In order to measure the behavior of a (relatively) small number of atoms, it is necessary to use a high vacuum environment, i.e. a volume having very little matter in it except for the few atoms being measured. Various techniques for obtaining such an appropriate vacuum are well known in the art and therefore will not be described further.
It is extremely helpful to have the longest amount of time possible to measure the energy levels (frequency) of atoms. One way to obtain a long measurement time is to keep the atoms in one place while measuring them. This can be done by putting the atoms in a cell or bottle; however, the internal kinetic motion (temperature) of the atoms causes them to collide with the walls of the bottle. Those collisions introduce frequency shifts in the energy level measurement.
Another way to measure the energy level of the atoms is to launch them in a "free-fall" trajectory such as an atomic beam. The relatively high speed of atoms in an atomic beam, typically V&gt;10.sup.4 cm/s at or above room temperature, limits the time available to make measurements to less than 0.002 sec. Nevertheless, the current generation of frequency standards use atomic clocks which are designed around the use of such an atomic beam.
Atomic clocks could be made more precise if the atoms moved more slowly when their atomic state is being measured. Since the temperature is proportional to the average of the square of the velocity (&lt;V.sup.2 &gt;), colder atoms mean slower moving atoms. Thus, the problem of making atoms move more slowly is a problem of reducing their temperature.
Cooling atoms in an atomic clock has been a problem for a long time. For example, in the 1950's, a researcher named Zacharias attempted to create a longer measurement time using an atomic fountain. Zacharias' idea was to direct a beam of "thermal" atoms in an upward direction within a chamber and then allow for the force of gravity to reduce their velocity. In principle, an atom can be made to move upward, stop and fall back (the principle is analogous to what happens to a football after kickoff). The period of time that an atom would spend at the top of its arc before returning to its starting position would provide for a longer measurement time. For an atom to follow such a ballistic trajectory, however, requires a low starting velocity, otherwise the atoms will cover far too much distance before gravity can bring them to a stop. More importantly, the atoms had to be cold, i.e., lack significant internal motions to keep upwardly directed beam of atoms from spreading out too far. Thus, while in principle Zacharias' "atomic fountain" could use Ramsey's method in making a high precision frequency standard (atomic clock) from a beam of atoms, it required a source of slow, cold atoms.
Zacharias' experiment stimulated several important developments in atomic physics--developments that ultimately led to the hydrogen maser and precise resonance experiments with bottled neutrons. The experiment with the atomic fountain itself, however, was a failure. Zacharias had hoped to avoid having to use a stream of thermal atoms in which the velocity of the atoms varies along what is known as a Boltzman distribution. A very small fraction of the atoms of a thermal beam move quite slowly. Zacharias hoped to select only these slow atoms for use in making his measurement; however, the faster atoms in the atomic fountain were found to scatter the slower atoms out of the beam and thereby make it impossible to obtain accurate measurements.
Thirty years later, in the 1980's, a technique for slowing atomic motion known as laser cooling was developed. Intense laser light normally causes matter to heat up and thus increase the random motion of the atoms. Under special conditions, however, it is possible to use pairs of laser beams properly positioned and operated to reduce atomic motion. This process is referred to as laser cooling.
One form of laser cooling uses a spherical quadrapole magnetic field and six laser beams aligned in pairs along each of three orthogonal axes to form an "optical trap." The effect that the light from these lasers has on atoms is a phenomena unique to atomic physics which has no analogy in daily experience. The light from each of the laser beams pushes the atoms harder when they are moving toward the laser than when they are moving away from the laser. The six lasers combine to prevent an atom from going anywhere and, indeed, from moving much at all, thus reducing its temperature and, in essence, creating a cooling process.
The six beams are circularly polarized such that, when they interact with the atoms in a magnetic field, they also force the atoms to collect in a small region of space in the center of the magnetic field coils. That process is described in an article by E. L. Raab, M. Prentiss, et al. in 59 Phys. Rev. Lett. 2631 (1987).
Attempts have been made to use very cold neutral atoms in experiments designed to make precise measurements of the microwave frequency "clock" transition of cesium and sodium atoms since, in principle, such an apparatus could be used to create a high resolution clock having small systematic errors. The results of those attempts, however, have been far from ideal. In order to carry out those experiments it has been necessary to use a large vacuum chamber, which is impractical. In addition, the signal to noise ratios generated during the experiments have so far been low, thus further reducing the utility of the concept. Consequently, the art has not yet produced a practical frequency standard using this type of laser cooling process for cooling the atoms.
A further problem arises in confining atoms using a magnetic field. Magnetically confined atoms have been considered unsuitable for use with the frequency standard of an atomic clock because variations in magnetic field strength produce variations in the transition frequency that "smear" the resulting spectrum. These variations can cause both irregularities in the frequency of the atomic clock, as well as render the magnetically confined atoms useless in an atomic clock. The strength of the magnetic field and the amount of resulting smearing also depend on the temperature of the atoms that are being confined. The internal kinetic motion of the atoms in a conventional atomic clock has required far too strong a magnetic field to attain confinement, and no attempt at reducing the kinetic motion through reduced temperature has been known to produce a useful transition.